In FFS (Form, Fill and Seal) machines, a continuous tubular strip of thermoplastic material housing an orderly succession of products is known, for example, from U.S. Pat. No. 6,574,944, to be fed along a feed path extending through a rotary ultrasound sealing unit located at a sealing station and which provides for closing the tubular strip along a succession of transverse seal lines, each located along a respective portion of the tubular strip extending between two adjacent products.
The known rotary unit referred to above comprises a first and second rotor, which are located on opposite sides of the feed path, have respective pitch surfaces tangent to each other and to the feed path, and rotate in opposite directions about a first and, respectively, second axis, which are parallel to each other and define a plane through the sealing station and perpendicular to the feed path.
The first rotor is a sealing rotor having a number of radial sealing heads equally spaced about the first axis and having respective sealing surfaces forming part of the respective pitch surface. The first rotor also has an actuating, ultrasound converting device coaxial with the first axis of rotation. The second rotor is an anvil rotor having a number of radially projecting anvils, a contrasting end surface of each of which forms part of the relative pitch surface and reaches the sealing station in time with a respective sealing surface.
The two rotors of the above known rotary unit are therefore completely different, and so react differently to the forces to which they are subjected, thus resulting in undesired in-service vibration which, combined with the vibration of the rotary unit itself, may result in impaired operating precision and, at times, in damage to either one of the rotors.